1. Field of the Invention
This present invention relates to a preparation of natural gas for its further transfer under pressure to a fuel tank of a vehicle, e.g., automobile, and may be used for providing individual gas-filling devices operated from a residential natural gas distribution network.
2. Description of the Related Art
Presently, there are used in this field gas-filling multistage compressors with both mechanical and hydraulic drives, which provide the compression of natural gas for its efficient application as a motor vehicle fuel. Complicated construction of compressors with mechanical drive, consumption of large amounts of power during their use, and generation of large amounts of heat, as well as high maintenance costs compensating a wear of movable parts of a compressor resulted in the development of compressors with hydraulic drives having some advantages over the compressors with mechanical drives.
It is known in the art a method for multistage compressing gas according to U.S. Pat. No. 5,863,186, wherein multistage gas compressing in series-connected compressing vessels of a compressor is performed by under-pressure supply of a hydraulic fluid thereinto, said hydraulic fluid being separated from the compressed gas by pistons moving in the vessels during operating cycles of the compressor. This method has found its application in gas-filling devices of ECOFUELER, including individual gas-filling appliances of HRA type (Home Refueling Appliance), operated from a residential low pressure gas network and from a standard residential electrical network (www.eco-fueler.com). The disadvantage of gas-filling devices operated according to this method is their high price limiting the broad use thereof in a private sector. The reason has to do with the need for high-technology constructional elements, mainly for precision hydraulic compressing vessels.
It is known in the art a method for hydraulic compression of gas for fueling a motor vehicle from mobile gas-filling appliances without a dividing piston between the gas and fluid (RU patent No. 2 128 803). The implementation of the method described in this patent provides the use of gas mainlines with gas pressure of 2.5 MPa (25 bar) and this method includes gas supply under said pressure into vertically arranged (because of the absence of the dividing piston) compressing vessels, compressing the gas and forcing it into accumulating vessels by an under-pressure supply of working fluid to the compressing vessels from an auxiliary vessel. To pump gas into the accumulating vessels there may be used two communicating compressing vessels, and gas accumulation in the accumulating vessel is performed by anti-phase alternate transfer from each compressing vessel of gas displaced from this vessel by fluid drawn from the other compressing vessel. The process of pumping the fluid from one vessel into the other is being performed by simultaneously filling the volume vacated by the fluid with gas from the gas mainline. The method described in RU patent No. 2 128 803 requires the observance of a condition that the ratio of the minimum volume of gas space in the working vessels to the volume between certain upper and lower levels of the fluid lies in the range from 1/20 to 1/25. This requirement is justified by “increase in operating and economical efficiency of one-stage gas compressing process” and is met by mounting of two—upper and lower—fluid-level sensors, so that once a certain upper level of the working fluid in a compressing vessel has been reached, a certain volume of non-displaced gas is left. Transfer of gas from accumulating vessels to User's vessels is performed by a displacement of fluid by gas with the sequential transfer of fluid from a previous vessel to the next ones. This method may be used in mobile gas-filling units providing large volumes of compressed gas by connection to a gas line with rather high pressure required for this method and having a power supply source of sufficient power (industrial electrical network). Moreover, because the above-mentioned condition provided by this method, when upon the termination of a compression cycle in a compressing vessel, a certain volume of compressed gas is left in its upper part, the effective volume of the further filling of a working vessel decreases due to significant volume expansion of this left non-displaced volume of the compressed gas. Therefore, the existence of such residual (“parasitic”) volume of compressed gas left in the working vessel at the end of a compression cycle results in the so called “stretched spring effect” at the stage of filling the compressing vessel (residual compressed gas begins to increase many fold in volume).
To summarize briefly the known methods for compressing natural gas for fueling motor vehicles, it may be seen that the technical level of solutions in this field is limited by two predominant variants, of which the first variant provides fueling a vehicle from a residential gas low pressure network at high costs of hardware, whereas the second variant cannot be used as an individual means for fueling motor vehicles with gas.